2003 Highlander - Heater Control Board questions - a REAL question

Hi all, recently had a thermostat replaced on my V6 which was stuck open.

Wife went to pick it up and "service advisor" stated and noted on invoice to
replace the Heater Control Board.  Quoted $300 if they rebuild or $900 if new.
Huh?

Called and questioned.  S/A said this is common problem when complaint of no
heat is received on these.

I made no complaint just wanted the thermostat R&R and that was it.  Why and
how did they test this?  Tech always tests and said insufficient heat was
coming out.

S/A repeated the tech tested and said it was bad and didn't know any more.  We
went round robin for awhile and finally S/A stated to try it for awhile and
see.

Car now warms up as normal and exhibits normal thermostat operation according
to temp gauge.

Heat control operates normally as does temperature control.

Questions:
1.    How does one test this?
2.    Is this a known failure?
3.    The price of a rebuild?  I got the impression they soldered same traces.
4.    Any other thoughts?

Thanks in advance,
--
Tekkie